Hexaaqua acidity

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

In all its complexes both neutral and ionic Ti111 is normally octahedral. In dilute acids the main species is [Ti(H20)6]3+, which hydrolyzes (pAT = 1) to give [Ti(0H)(H20)5]2+. The hexaaqua ion occurs in alums such as CsTi(S04)2-12H20 and in simple salts such as [Ti(H20)6](/ -MeC6H4S03)3 3H20.54... 

The divalent state is the common and most stable oxidation state. In neutral or acid aqueous solution there is the very pale pink hexaaqua ion [Mn(H20)6]2, Which is resistant to oxidation as shown by the potentials... 

Glowiak, T., Dao, C.N., Legendziewicz, J., and Huskowska, E. (1991) Structures of neodymium(III) complexes with amino acids (1) catena-poly[[triaquatrichloroneodymium(lll)]-p-(f -alanine-0,0 ) (II) pentaaquachloro-p-(L-proline-0,0 )-neodymium(III) hexaaqua-p-(L-proUne-0,0 )-neodymium(III) pen-tachloride. Acta Crystallographica Section C, 47 (1), 78-81. 

Figure 6.5b shows another representation of a hexaaqua ion. Each O atom donates a pair of electrons to the metal M"+ ion, and each H2O molecule acts as a Lewis base while the metal ion functions as a Lewis acid. We are implying that the M—O interaction is essentially covalent, in contrast to the case for Na in Figure 6.5a. In practice, the character of the metal oxygen interaction varies with the nature of the metal ion and relevant to this is the electroneutrality principle (see Section 19.6).

It is instructive to compare acid strengths of hexaaqua ions with other acids. The values of MeC02H (equation... 

Harris and co-workers have extensively studied the kinetics of water substitutions in [Rh(H20)6] + (equation 281 X = HjO," Cl, " Br " ). A two-term rate law was found in all three cases, as was an inverse [H" ] effect. Reaction of the substituting ligand with either [Rh(H20)6], or its conjugate base [Rh(H20)50H], accounts for the two-term rate law, while the inverse [H ] effect occurs because the [Rh(H20)0H] is several orders of magnitude more reactive than the hexaaqua ion, making the base-catalyzed path the dominant route in all but the most acidic solutions. For Cl and Br anations, the monohalo-substituted species were not isolated, as rapid substitution of a second halide leads to the (presumably trans) [Rh(H20)4X2] ions. The kinetic irons effect was also cited to explain the reactivity of the [Rh(H20)50Hp ion for the [Rh(H30)5X] series, the order of the kinetic trans effect induced by X was shown to be OH > Br > Cl." Kinetic studies of the anation of [Rh(H20)4] by NCS (equation 281 X = NCS ) have been presented by Filipenko and co-workers at 80 °C, = 0.104 M s ,... 

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